The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Two WA firefighters still critical

Two women firefighters who suffered life threatening burns remain in a critical condition in Royal Perth Hospital.

A spokesman for the hospital said on Saturday both had been badly burnt, with one in a critical condition and the other in a critical but stable condition.

WA Fire and Emergency Services Authority (FESA) state duty director Bruce Jones said the firefighters were from the Department of Environment and Conservation and the local bushfire brigade who were responding to a fire in a pine plantation at Albany on Friday night.

"Preliminary reports state that the firefighters were caught when strong winds caused the fire to suddenly change direction on a slope," he said in a statement.

The Royal Flying Doctor Service despatched two aircraft to Albany to transport the two women to Perth.

One aged 45 suffered burns to 60 per cent of her body and other aged 24 suffered burns to 40 per cent of her body, a Royal Flying Doctor Service spokeswoman said.

The incident will be investigated by fire investigators, police and other authorities.